The art has long sought to determine atmospheric disturbances which might adversely affect persons and property in or along the path of that disturbance. For example, the U. S. Forest Service has attempted to determine disturbances in the atmosphere which indicate the presence of a forest fire at some distant location. The U. S. Weather Service has attempted to detect the presence and movement of discontinuities in the atmosphere which indicate the presence or the likelihood of some hazardous condition, such as heavy rainfalls, hurricanes, tornadoes and the like. The U. S. Federal Aeronautics Administration has attempted to detect the presence or likelihood of discontinuities in the atmosphere in and around airports, which discontinuities indicate the presence or likelihood of weather conditions hazardous to aircraft landing or taking off from an airport, such as squall lines, wind shears and the like.
A number of different approaches for detection of such disturbances has been proposed in the art, but each of the approaches has its own advantages and disadvantages. For example, satellite pictures are useful in determining such hazards, but those pictures show only developed hazards and cannot show the likelihood of those hazards developing, especially in connection with transitory or very localized disturbances, such as tornadoes. Ground-observed or balloon-observed disturbances are detected, but in order to accurately detect and map the progress of such disturbances, a great number of such observations are required, and in many locations, such number of observations is simply not practical.
These difficulties in detecting such disturbances are particularly disadvantageous where the disturbances present imminent danger to property or persons, such as persons aboard an aircraft taking off or landing at airports. In this regard, the art has sought to detect the presence or imminent formation of such disturbances by a variety of methods, operated either from ground installations or from aircraft. Radar is a very common technique for such detection, but radar is able to detect, primarily, only developed hazards and then only when the developed hazards have sufficient reflectivity to make radar detection accurate. In addition, such radar installations are quite expensive, both for initial installation and for operation and maintenance, which makes such installations unsuitable for lower volume airports, in view of the cost thereof.
For a number of years, the art has sought to provide passive infrared radiation detection of such disturbances. Infrared radiation detection equipment is far less expensive than radar, for example, in terms of initial installation, maintenance and operation, but prior art infrared detection systems have suffered from considerable disadvantages. For example, an existing or developing wind shear near an airport can be detected by passive infrared spectrometry due to the difference in temperature between the ambient atmospheric air and the cooler air forming or about to form a wind shear. However, in order to make such detection accurate, it is necessary to very accurately measure the temperature of air at various azimuths, elevations and distances from the observation point. Particularly, for wind shear, that accuracy must be, preferably, within several degrees centigrade, and at least within about 8.degree. to 10.degree. C., or otherwise developing or present wind shears will not be detected or substantial numbers of false detections will occur.
One of the earliest passive infrared detection devices is disclosed in U.S. Pat. No. 3,103,586, issued on Sep. 10, 1963. This patent points out that prior methods for determining the distance or range of an object, e.g. an aircraft, by use of radar or radio-direction finding equipment but such techniques are susceptible to a variety of jamming techniques, as might be practiced, for example, by an enemy aircraft. The patent proposes that such an object, e.g. an aircraft, be detected and ranged by use of infrared radiation, emanating from that object, e.g. an aircraft, itself. The patent points out that such ranging method depends upon a determination of the concentration and distribution of infrared adsorbing gases in the air, but that the major constituents of air, i.e. oxygen and nitrogen, do not have absorption bands in the infrared. Therefore, the patent points out that the two other major gases in atmospheric air, i.e. carbon dioxide and water vapor, can be used for such ranging method. The method is applicable where the object of interest, or target, emits heat, and, hence infrared energy. The distance or ranging of that object can be theoretically determined from the fraction of infrared energy adsorbed by carbon dioxide or water vapor in the path of observation to the aircraft.
It is quite obvious, however, that in order for such a system to be accurate, the concentration and distribution of the carbon dioxide and water vapor in the path of observation must be known. Fortunately, as pointed out in that patent, the concentration and distribution of carbon dioxide in the atmosphere is relatively constant up through about 100,000 feet. However, the concentration and distribution of water vapor in the atmosphere is quite variable. Therefore, unless one knows the concentration and distribution of water vapor in the path of observation, accurate ranging cannot be accomplished by infrared absorption of water vapor. For this reason, the art has concentrated on such ranging techniques by infrared absorption of carbon dioxide, since the concentration thereof is relatively constant. This approach, nevertheless, has the inherent accuracy of the absorption of infrared energy in the path of observation caused by water vapor therein.
U.S. Pat. No. 3,117,228, issued on Jan. 7, 1964, discloses that the accuracy of such ranging method, based on carbon dioxide absorption, may be improved. That improved accuracy is based on the discovery that there are spectral bands in which carbon dioxide preferentially absorbs infrared radiation, as opposed to other spectral bands. Thus, that patent proposes determining the distance of a hot object by detecting the infrared radiation from that hot object, e.g. an aircraft, by utilizing the property of selective atmospheric infrared attenuation.
With the realization that microbursts or wind shears are causes of past tragic aircraft crashes, particularly when landing or taking off from an airport, considerable effort was expended in the art to adapt the ranging techniques, discussed above, to the detection of microbursts or wind shears ahead of an aircraft or near an airport. Thus, U.S. Pat. No. 4,342,912, issued on Aug. 3, 1982, discloses that air disturbances created by low level wind shear can be detected by air temperature gradients existing at different distances from the observer, e.g. an aircraft or airport. That patent points out that when using filters having different infrared frequency band passes, the effective sensed distance is different for each filter. Hence, the device is thus able to sequentially sense temperatures at different distances and, more particularly, sense temperature variations at different distances. Such different sensed temperatures can, therefore, be used to detect microbursts and wind shear. However, like the prior art, this method is also based on absorption by carbon dioxide and suffers from the disadvantage of the errors introduced by the infrared absorption of water vapor.
U.S. Pat. No. 4,937,447, issued on Jun. 26, 1990, improves upon the method of the above-discussed patent in the discovery that two infrared bands of naturally occurring carbon dioxide, i.e. in the 13 to 15 micron region and in the 3 to 5 micron region, provide different sensitivities. In, particularly, the 4.15 to about 4.2 micron band, mainly, regularly spaced carbon dioxide trace lines, free of interfering spectral lines of nitrous oxide, methane, ozone, and water vapor, are found. Accordingly, temperature differences of a column of atmospheric air and ambient air are obtained by sequentially sensing the intensity of at least two spectral peaks in the 4.17 to 4.2 micron region. Changes in the relative intensity of the spectral peaks indicate a change in the temperature of air within the column of atmospheric air and ambient air. This change in temperature is indicative of an air disturbance, such as clear air turbulence or wind shear. A Fabry-Perot device is used for sequentially sensing the preselected wavelengths.
U.S. Pat. No. 4,965,572 points out that even though such infrared detecting devices, as described above, can measure air temperature gradients associated with wind shear, and the like, based on carbon dioxide absorption, the conclusion of a wind shear on the basis of the determined temperature gradients alone will not provide the degree of accuracy and reliability desired, in terms of minimizing both the number of instances of wind shear that are missed and the number of instances where wind shear is falsely indicated. That patent, therefore, proposed incorporating the data from such infrared detecting methods with other data associated with the aircraft for determining a hazard factor which is indicative of possible wind shear. To improve existing infrared sensing devices, the patent also proposes the use of a multi-spectral infrared spectrometer for controlling the infrared frequency band pass and, thus, the effective sensed distance.
U.S. Pat. No. 4,965,573, issued on Oct. 23, 1990, seeks to further improve the detection of wind shear and the like, by use of a scanning, multi-spectral radiometer which sweeps an approximate 60.degree. path in front of an aircraft. The radiometer employs two rows of detectors that are slightly offset, i.e. about 7.degree. apart in elevation, resulting in two simultaneous measurements of IT attenuation, based on carbon dioxide absorption. That dual information allows the continuous measurement of atmospheric vertical temperature gradients, or lapse rate, for use in determining atmospheric stability and, hence, the probability of microburst occurrence.
While using carbon dioxide absorption as the basis of the method, this patent acknowledges that water vapor, in the atmosphere under observation, may cause errors in the measured absorption/extinction characteristics. To mitigate these errors, the patent uses two filters (and one reference filter) for measurement of a lapse rate when the humidity is low and a different set of two filters (and one reference filter) when the humidity is high. For this selection of sets of filters, the humidity at the aircraft is measured, and this information is used to determine which set of two filters and a reference filter to use. This method, however, assumes that the humidity along the entire line of sight to the wind shear (impending or in progress) is the same as the humidity measured at the aircraft. This assumption may be valid for short distances, e.g. 1 Km, but it is not valid for long distances, e.g. 20 Km. This assumption, also, leads to 10 to 20% error in range calculations (as indicated in the patent) due to uncertainty in the humidity distribution. Such errors are quite unacceptable for any reasonable method for establishing overall weather conditions and usually unacceptable for attempting to accurately locate local weather disturbances. For example, in mapping weather at ranges up to 20 Km, a 20% error will distort the map by 200 meters at 1 Km and 4 Km at 20 Km; this is an unacceptable error.
Thus, all of the prior methods for remote sensing and ranging of atmospheric air temperatures, as discussed above, are necessarily based on the assumption that the water vapor which exists between the point of observation and the weather event under observation is either constant along the entire path of observation or is insignificant. In reality, however, for example, water vapor in a 360.degree. observation and at up to a 20 Km radius for mapping purposes, is neither constant nor insignificant.
In commonly used absorption ranges, such as 10 to 14 microns, the water vapor may effect the absorption extinction as much as or even more than the carbon dioxide itself (depending upon the humidity levels), and, unlike carbon dioxide, water vapor is not evenly distributed in the atmosphere. Not considering the water vapor content can result in very substantial inaccuracies in both range and temperature calculations. For example, a 2 Km localized patch of high humidity in the direct line of sight under observation can lead to erroneous air temperatures, based on IR extinction of carbon dioxide alone, by as much as 35.degree. C. (where a 10.degree. to 20.degree. C. change in temperature gradient is enough to cause very serious weather events). This means that the prior art techniques, discussed above, are correct only in the special cases of no (or very low) humidity, or where the local humidity can be validly assumed to be the humidity along the path of observation. In all other cases, erroneous measurements, impaired detection probability and false alarms will result.
Unfortunately, while the art appreciated the above-discussed significance of water vapor in such determinations, the art could find no practical way of accounting for or calculating the water vapor in the path of observation. Under the circumstances, all of these prior art methods have been found to be too inaccurate for use in weather mapping at significant ranges or for accurately determining weather events.
Further, for any practical weather mapping, temperature, alone, is not sufficient, and a practical weather map must include humidity. Accordingly, these prior art methods are not useful for producing a weather map.
It would, therefore, be of substantial advantage to the art to provide a method with which the accuracy of such infrared measurements of atmospheric conditions is substantially improved and, particularly, where the water vapor content of a path of observation can be determined with distance so as to very substantially improve the accuracy of the determinations and provide weather mapping.